1. Field of the Invention
The invention is directed to a method and apparatus for determining concentration of a gas, and particularly for calibrating a gas detection device or verifying the calibration of a diffusion limited gas detection device.
2. Description of Related Art
The technology of gas detection by means of sensors is well known in the art. There are many types of sensors available designed for various applications; one of the most common types of sensors is the diffusion limited electrochemical gas sensor. These sensors have been described, for example, in Lauer, U.S. Pat. No. 3,767,552, Oswin et al, U.S. Pat. Nos. 3,824,168, 3,909,386 and 3,992,267, and Tantram et al, U.S. Pat. Nos. 4,132,616 and 4,324,632, and are widely used for measuring oxygen concentrations in the air and for toxic gas detection for work-place safety, emission monitoring and control of pollutants, and performance optimization of combustion engines.
Most electrochemical amperometric sensors are designed such that the gas to be detected is either oxidized or reduced at an electrode within the sensor, and the gas typically passes through one or more diffusion barriers, such as a porous membrane, capillary sintered disk, etc. to reach the electrode. The majority of sensors are operated such that the response current is limited by the rate at which the gas can diffuse into the sensor. A diffusion limited sensor has the advantageous properties of a linear response to the gas concentration, stable output with small changes in operating potential due to environmental or instrumental changes (e.g. variations in power supply voltage), and either a small or at least well defined variation in output with temperature and pressure. The theory of operation and practical usage of electrochemical sensors has been discussed in detail by S. C. Chang, J. R. Stetter, C. S. Cha, Talanta, Amperometric Gas Sensors (1993), 40, 461 and by B. S. Hobbs, A. D. S. Tantram, R. Chan-Henry in Techniques and Mechanisms in Gas Sensing, Ed. P. T. Mosely, J. Norris, D. E. Williams, (1991).
In a typical design, a sensor will contain two or more electrodes within the sensor in contact with an ionically conductive electrolyte. One electrode is designated the sensing electrode and it is located behind a gas porous or gas permeable membrane. The gas to be detected enters the sensor and moves by diffusion through the membrane and any other diffusion barriers in the gas path to the electrode. The gas is consumed at the electrode in either an oxidation or a reduction process, and the resulting electrical charge passes from the electrode, through the external circuit to the counter electrode. The magnitude of this electric current provides the output signal. At the counter electrode, which must also be in contact with the electrolyte, an equal and opposite electrochemical reaction occurs. Thus, if entry of the gas into the sensor results in an oxidation reaction at the working electrode, then there must be an equal magnitude reduction at the counter electrode; similarly, if the gas produces a reduction at the working electrode, then there must be an equal magnitude oxidation at the counter electrode.
The output signal from an amperometric sensor in the presence of the gas to be detected is determined by the gas concentration, and by the diffusivity of the gas path through which the gas must pass to reach the sensing electrode. The diffusivity is defined here as a measure of how much gas at unit concentration will diffuse into the sensor per second is further defined in mathematical form below. If the diffusivity of the sensor were known, then the gas concentration could be calculated upon measuring the out current from the sensor. However, variations in the physical properties of the components used to manufacture electrochemical sensors, variations which occur during the during the operational lifetime of the sensor and the difficulties in obtaining reliable diffusion parameters for the components comprising the sensor gas path preclude this method from being used in practice.
Therefore, it is common practice for a sensor to be calibrated with a known concentration of test gas after installation into a gas detection instrument. The calibration procedure typically involves application of the test gas for sufficient time for the output signal to reach steady state, after which the instrument equates the nominal gas concentration with the output signal from the sensor. This procedure allows, for example, an instrument with a numeric display to give a visual indication of the gas concentration in common gas concentration units (e.g. mg/m.sup.3, ppm, % volume). Since the output current from the sensor may vary with time, it is common practice to periodically re-calibrate the sensor; the frequency of this action is determined by the nature of the sensor and the accuracy requirements of the application. Thus for a work-pace safety application, an instrument for carbon monoxide may require calibration monthly, whereas an oxygen sensor in a medical critical care application may require more frequent calibration.
Many locations where gas detection instruments are installed are difficult to reach, or pose other problems to achieve calibration such as their locations being classified as hazardous by the National Electrical Code (NFPA 70--National Electrical Code--1996 Edition). As another example, calibration is likely to be done infrequently, if at all, in residential gas detection devices, such those introduced recently for carbon monoxide by several manufacturers, as described for example by Sneider et al. in U.S. Pat. No. 5,667,653, Stetter in U.S. Pat. No. 5,331,310 and Goldstein in U.S. Pat. Nos. 5,063,164 and 5,618,493. Even if periodic calibration of the sensor in the instrument is performed, the sensor may still fail during the time interval between calibrations, and this failure will result in false gas concentration readings, or failure to respond when exposed to the gas. Therefore, it is desirable to have a means to determine whether the output signal from the sensor is a valid measure of the gas concentration.
The problems outlined above have been addressed in the prior art to various levels of satisfaction. For example, Stetter et al in U.S. Pat. No. 4,384,925 describes a method for automatic calibration of a fixed point gas detection instrument including a calibration flow system which connects cylinders of test gas to the sensor controlled by a microprocessor. Periodically, the instrument determines if the response of the sensor is within the specified limits. Hyer and Roberts in U.S. Pat. No. 4,151,738, Hartwig and Habibi in U.S. Pat. No. 5,239,492 and Melgaard in U.S. Pat. No. 4,116,612 also describe calibration systems, where the delivery of the calibration gas to the gas detector is controlled by a microprocessor or other automated system.
Electrochemical gas generators have been used by Analytical Technology Inc. of Oaks, Pa. 19456 (8 Page Technical Information Sheet, titled A world of gases . . . A single transmitter) to provide test gas to automatically check the performance of gas detection instruments, and ensure that the sensors are responding within their specified limits. Finbow et al. describe in U.S. Pat. No. 5,668,302 incorporating an electrochemical gas generator within an electrochemical gas sensor, behind the diffusion barrier, to provide a means for automatic function testing of the gas detection instrument. The calibration methods though are limited by the accuracy by which the electrochemical gas generators can reliably and repetitively produce a test gas of known concentration, and in the latter example will not be able to identify a blocked gas diffusion path as being a fault.
In one example in the prior art, described by Capetanopoulos in European Pat. Application No. 663,594, the response time of the sensor to a change in the external diffusion barrier was measured and the concentration of an unknown gas concentration was found by comparison to that of the response time to a change in the diffusion barrier with a known test gas concentration. However, this method, though novel, still relied upon calibration with a test gas of known concentration and involves a complex analysis based on the time response of the sensor, and so is more prone to experimental error than measuring the response at steady state.
All the methods described above rely on the presence of a test gas of known concentration. For oxygen sensors, ambient air is often used, since the concentration in well ventilated areas is a constant 20.9 volume percent; however for oxygen sensors in poorly ventilated areas and for sensors for other gases, a compressed gas cylinder or other means of producing known test gas concentrations are required to do the calibration.
A method described by Tantram and Gilbey in U.S. Pat. No. 4,829,809 does not require a known test gas concentration. A test gas of unknown concentration is passed over the sensor through a calibration flow system of known volume. After flushing the system, the gas flow is stopped and the calibration system is sealed. The output current decays to zero as the sensor consumes the gas, and the sensitivity of the initial gas concentration can be found, using Faraday's law from the integral of the current, i.e. from the total charge passed, and the volume of the sealed flow system. Waiting for the current to decay exponentially to zero is a long process and subject to errors, especially if there is a non-gas related background current, so the inventors devised a quicker analysis method involving sampling the current at various times after the start of the test, and calculating the total charge passed. This method is noteworthy because it provides a method of calibrating the sensor without requiring prior knowledge of the concentration of the test gas. However, the method is susceptible to errors due to leakage and poor mixing of the gas within the calibration flow system.
A similar system has been described by Matthiesen in U.S. Pat. No. 4,833,909 for using an electrochemical sensor to coulometrically determine a gas concentration.
Other approaches have focused on the electrical properties of the sensor. For example, Jones in U.S. Pat. No. 5,202,637 and Studer in U.S. Pat. No. 5,611,909 apply a small potential perturbation to the normally constant potential between the reference electrode and the working electrode and monitor the electrical current response of the sensor. While certainly providing a simple in-situ test that an instrument or controller can automatically perform on the sensor, this method will only detect those modes of sensor failure which affect the electrical properties of the working electrode, such as loss of volume due to dry-out from an aqueous based electrolyte. This test is unable to detect sensor faults due to problems which do not affect the electrical properties of the working electrode, for example, blockage of the gas path by dust or condensation.
Doer and Linowski have also described electrical tests for HPLC electrochemical detectors in U.S. Pat. No. 5,100,530.